Transport of Public Switched Telephone Network (PSTN) text phone (TTY) signals over packet switched networks is a relatively new concept and one studied only recently within various standards bodies. The International Telecommunication Union (ITU) and Telecommunications Industry Association (TIA) are working on International and United States (US) standards, respectively, to facilitate the transport of text phone signals received by a Voice Over Internet Protocol (VoIP) gateway from a PSTN text phone over an Internet Protocol (IP) network within the context of a “VoIP call”.
New TIA 1001 and ITU V.151 standards, and text relay protocol (RFC 2793), describe how to transport text over IP networks. Initial proposals required that every PSTN text phone be supported by every gateway, which is expensive and will take years to realize. Alternative proposals recommend using only a subset of text phone protocols, allowing only like TTY devices to communicate as they do today over the PSTN. There is a desire for the gateways to provide interworking functionality between dissimilar TTY types through gateways connected to the PSTN.
The Text Relay (TR) protocol requires both IP endpoints to negotiate a text relay media capability. The IP endpoint detecting a text terminal call has to elect to use the TR protocol in response to specific stimulus from the text phone signal. This TR decision is based on IP endpoint call discrimination procedures. Unlike an audio media format where TTY signals are carried as audio signals, the text relay media format cannot be used unless a device knows that the remote device can properly deliver the text information received to the user. Thus, the text relay capability is normally confined to media gateways connected to like TTY devices on each end and to IP phones with RFC 2793 text capabilities.
Problems arise trying to provide interworking between dissimilar PSTN text phone modulations. A gateway cannot simply shift to text relay without also potentially sacrificing interoperability between like PSTN text phone devices. For example, a gateway in the United Kingdom (UK) may only support an ITU V.21 text modulation and a gateway in the United States (US) may only support Baudot modulation.
A caller in the UK could call the US and the UK gateway may then propose a shift from the audio mode to the text relay mode. The caller in the UK could then start sending text packets to the gateway in the US when the called party in the US is actually using a V.21 text terminal. This would preclude the users from communicating since the US gateway cannot convert the text in the text relay mode packets back into PSTN V.21 modulation tones.
The present invention addresses this and other problems associated with the prior art.